Yeah, all those high school shows especially set around the 2000s had actors who were older. I've always thought that part of the reason was that these shows had all these teenagers having sex and they felt better about them looking older.

As far as Smallville goes, I believe Allison Mack, Kristin Kreuk and Sam Jones (who played Pete) were all 18 when the show started but that's as young as it got.